The invention relates generally to heating systems, and more specifically, to multi-source heating systems.
A relatively recent innovation in heating systems has been the introduction of multi-source heating systems. Several such systems use a heat pump as a primary heating source. Auxiliary sources are then provided to add heat during times when the heat pump does not function efficiently. Some typical auxiliary sources include fossil fuel furnaces (i.e. fuel oil, propane or natural gas furnaces, for example), electric heating strips, as well as other heat sources.
As is known, heat pumps may be used to heat an interior space by transporting heat from an exterior environment to an interior space. Air-to-air heat pumps operate to transport heat contained in outside air to warm the interior space. Water-to-air heat pumps operate to transport heat from water (for example groundwater or a nearby pond or other water source) to warm an interior space. Ground source heat pumps operate to transport heat from the ground to an interior space. In each system, heat transfer is typically achieved by control of a liquid/gas state change of a refrigerant.
In many cases, the phase change from a liquid to a gas is induced in the refrigerant in an evaporator. The gaseous refrigerant passes through an external heat exchanger, picking up heat from the external air, water or ground, depending on the type of heat pump. The gaseous refrigerant is then compressed into a higher temperature liquid in a compressor. The hot liquid refrigerant is then passed through an internal heat exchanger. Air in a ventilation system is forced over the internal heat exchanger and absorbs heat from the heated liquid refrigerant. The cooler liquid refrigerant then returns to the evaporator and is recycled back through the system.
The net result of this cycle is the transportation of heat from the colder exterior medium to warm the interior air. The amount of electrical energy required to transport this heat (the electrical power consumption of the compressor and the interior and exterior fans) is generally less than the electrical energy equivalent of the transported heat. Thus, a heat pump, when operating at or near optimal conditions can often provide more efficient heating than an electric resistance heater, for example, using the same amount of electrical power.
The efficiency of a heat pump typically degrades as the outside temperature falls. In some cases, the external heat exchanger for an air-to-air heat pump (which is a common type of heat pump) cannot sufficiently “warm” the refrigerant if the outside air temperature is too low. For example, in some cases, the cold liquid entering the external heat exchanger may be on the order of 0 degrees Fahrenheit. If the outside air temperature is below 0 degrees Fahrenheit, heat absorption by the refrigerant will be minimal at best. As can be seen, the efficiency of an air-to-air heat type pump may go down with outside temperature. In some cases, frost can even develop on an external heat exchanger, creating additional difficulty by slowing heat exchange and potentially damaging the equipment.
Because of these and other shortcomings of heat pumps, some heating systems use an auxiliary heat source to provide heat when the heat pump is not functioning efficiently. The auxiliary heat source may be activated when, for example, the heat pump no longer has the capacity to heat the inside space. In many cases, the auxiliary heat source may have a greater heat producing capacity than the heat pump, but typically cost more to operate.
In some cases, an auxiliary heat source is added or retrofitted to an existing heating system. Typically, the auxiliary heat source is included with an auxiliary control kit that receives an on/off signal or the like from the controller of the existing heating system. In some cases, the controller of the existing heating system can provide calls for two or more stages of heating. The inclusion of the auxiliary control kit, however, adds expense to the system. In addition, the auxiliary control kit may itself include internal controls that prevent the controller of the existing system from fully controlling the auxiliary heat source.